The present invention relates to a focus detection apparatus for use in adjusting the focus of an image-pickup optical system in an image-pickup apparatus such as a digital camera, a video camera, and a film camera, and more particularly, to a focus detection apparatus in a phase difference detection method.
Focus detection methods include a so-called contrast detection method in which a video signal is produced by an image-pickup element and is used for focus detection. The focus detection of the contrast detection method involves a search for an in-focus position corresponding to the peak of the contrast while a focus lens of an image-pickup optical system is gradually moved on an optical axis, so that the method tends to require a long time to achieve an in-focus state.
For this reason, focus detection is often performed with a phase difference detection method in a single-lens reflex digital camera. In the phase difference detection method, since a necessary moving distance of a focus lens for achieving an in-focus state can be determined from the defocus amount of an image-pickup optical system, the time taken to achieve the in-focus state can be significantly reduced as compared with the contrast detection method.
In the focus detection of the phase difference detection method, multipoint focus detection has been proposed in which a focus detection area is provided not only at the center of an image-pickup range but also in an off-axis area on the periphery of the image-pickup range. Japanese Patent Laid-Open No. 8(1996)-262320 is an example of such a focus detection method and has disclosed a focus detection apparatus which performs focus detection in an off-axis area within an image-pickup range.
In the focus detection apparatus disclosed in Japanese Patent Laid-Open No. 8(1996)-262320, phase difference detection in a horizontal direction in the off-axis area is performed with a pair of images formed on a pair of light-receiving element arrays by a pair of lens portions formed on an entrance surface of a secondary image-forming lens and a pair of prism portions formed on an emergence surface thereof. Defocus of an image-pickup lens moves the paired images in the horizontal direction in which a pair of apertures in a stop is arranged. The moving direction of the images is referred to as a correlation direction.
When the lens portion on the entrance side of the secondary image-forming lens and the prism portion on the emergence side thereof are collectively regarded as one optical system, the paired optical systems for detecting a phase difference in the horizontal direction are asymmetric not only in the horizontal direction serving as the correlation direction but also in the vertical direction.
In the paired optical systems asymmetric not only in the correlation direction (horizontal direction) but also in the vertical direction, deformation of the secondary image-forming lens due to a change in ambient environments such as temperature and humidity causes the paired images on the light-receiving element arrays to be moved in both of the horizontal direction and the vertical direction. The moving distance of one of the paired images is different from that of the other since the paired optical systems are asymmetric.
Some proposals have been made on a method of correcting movements of the paired images having different moving distances in the correlation direction resulting from a change in ambient environments by providing sensors which detect temperature and humidity. Even when the correction is performed in the correlation direction, movements of the paired images in the vertical direction orthogonal to the correlation direction may lead to varying focus detection results depending on an object to reduce the accuracy of focus detection.
The focus detection apparatus disclosed in Japanese Patent Laid-Open No. 8(1996)-262320 includes the light-receiving element arrays arranged only in one-dimensional direction in the off-axis area. There is a possibility that focus detection depending on the contrast component of an object cannot be performed.